Roughness can be measured by manual comparison against a "surface roughness comparator" (a sample of known surface roughness), but more generally a surface profile measurement is made with a profilometer. These can be of the contact variety (typically a diamond stylus) or optical (e.g.: a white light interferometer or laser scanning confocal microscope).
The contour centerline M is the reference line for evaluating the surface roughness value
(2) Ten points height Rz of micro unevenness
(3) Maximum contour height Ry